The present invention relates generally to filters, and more particularly to two-pole monolithic crystal filters having improved coupling and attenuation characteristics.
Prior art monolithic crystal filters, as illustrated in FIGS. 1 and 2, typically include pairs of opposingly disposed electrodes plated on opposite surfaces of a quartz crystal wafer 100. The monolithic crystal filter may include pairs of electrodes 101, 103, and 102, 104 as in FIG. 1, or may include separate input and output electrodes 105 and 106 and a common electrode 107 as in FIG. 2. The basic electrical characteristics of such monolithic crystal filters are generally described in William D. Beaver's PhD Dissertation entitled, "Theory and Design Principles of the Monolithic Crystal Filter", Lehigh University, 1967.
The acoustical coupling between the input and output electrodes of a monolithic crystal filter is dependent upon, among other things, the distance between the electrodes along the axis of coupling, the length of the sides of the electrodes parallel to the axis of coupling and the difference in the resonating frequency between the plated and the unplated surfaces of the crystal wafer, which is typically referred to as the plateback. In order to resonate at higher frequencies, the electrodes of the monolithic crystal filter may be designed to operate in an overtone mode of oscillation. However, in order to provide adequate coupling in an overtone mode filter, the input and output electrodes must be arranged extremely close together. But, as the input and output electrodes are moved closer together, problems are encountered both in manufacturing such overtone mode filters due to tolerance variations and in degradation of the attenuation characteristics of such overtone mode filters due to the capacitance between the input and output electrodes.
The degradation in the attenuation characteristics of overtone mode filters may be lessened somewhat by overlapping the electrodes as described in U.S. Pat. No. 4,163,959. According to another prior monolithic crystal filter arrangement, the capacitance between the input and output electrode may be reduced by placing additional grounded electrodes between the input and output electrodes. But, such additional electrodes must have the same resonating frequency as the input and output electrodes. It has also been demonstrated for another prior monolithic crystal filter arrangement that the acoustical coupling between the input and output electrodes can be vernier adjusted by plating the interelectrode region. However, none of the foregoing monolithic crystal filter arrangements provide viable solutions for the problems created due to the extremely close spacing required between the input and the output electrodes for overtone mode operation.